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The Chemistry of FoodLecture 1

Chemistry in Context06525/06529/06509

Dr AN Boa

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Topics in Food Chemistry

Physical and chemical properties of:

• Carbohydrates– saccharides (mono-, di-, oligo- and poly-)

• Proteins– amino acids and polypeptides

• Lipids– oils and fats

• Colourings• Flavour chemicals• Vitamins• Minerals• Water• Preservatives

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Carbohydrates - general• Source of energy and sweetness (see later)

• Carbohydrates consist of single units known as monosaccharides or multiple units thereof.

• Simple sugars (mono- and disaccharides)• Oligo- and polysaccharides (sugar polymers)

• Many stereoisomers possible

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Polarimetry

light source polarizersample cell

analyzing polarizermonochromator

α

c

l

c

α

Observer rotates analyzing polarizer to view maximum / minimum light and records angle α

Length of sample cell l (dm) is known, concentration c (g.cm-3) may be known or unknown

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Simple Sugars• Glucose

– also known as grape sugar, corn sugar or dextrose– dextrorotatory (+)– made commercially by hydrolyzing starch

• Fructose– also known as fruit sugar or laevulose– laevorotatory (-) (but in the D-sugar series!)– made by hydrolyzing starch and isomerizing the

glucose from the corn syrup

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Monosaccharides

• General Formula (CH2O)n

• Polyhydroxy carbonyl compounds, e.g. aldoses(aldehyde based) and ketoses (ketone based)

• Carbon content can vary (typically 3-8) for each type of saccharide, e.g. pentoses (C5) and hexoses(C6)

FISCHER PROJECTIONS

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• Monosaccharides mainly exist as cyclic hemiacetals• Attributed names on the basis of the ring size, e.g.

furanose (5-ring, c.f. furan) and pyranose (6 ring, c.f. pyran)

• α- and β- anomers of the cyclic forms exist

Monosaccharides

hemiacetal

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Cyclisation of linear sugars

OH

HO

OH O

HO

H

OHO

O5

5

5

5 1

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1

α-anomer β-anomer

Carbonyl carbon freely rotates→O can attack either

face of C=O

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The anomeric effect• When compared to the β-anomer, the α-anomer is often much more stable than

expected so that in many cases [α-anomer] > [β-anomer].

• There is a stereoelectronic preference for conformations in which the best donor lone pair, or bond, is antiperiplanar to the best acceptor bond.

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Monosaccharides• Fructose

Fischer projection Haworth projection

• Conversion of either anomer into the equilibrium mixture is called mutarotation

+

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Mutarotation

• Single molecule in solution can interchange between– straight chain and ring– different ring sizes– α and β anomers

• Mutarotation is– a dynamic equilibrium– all isomers can potentially exist in solution– energy / stability of different forms vary

straight chain

α-furanose β-furanose

α-pyranose β-pyranose

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Sucrose and Lactose: disaccharides

• Sucrose– Also known as cane sugar or beet sugar– Obtained commercially from sugar beet or sugar cane– Only one isomer present in solution– Easily crystallized

• Lactose– Also known as milk sugar– Primary sugar in mammalian milk (human > cow)– Also is a by-product of cheese manufacture– Not very sweet (relative intensity ≤ 0.3 cf sucrose)– Hydrolyzed by enzyme β-galactosidase. Many adults lack

this enzyme and are therefore lactose intolerant

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Glycosidic linkages• Disaccharides are formed by linking two monosaccharides

via a glycosidic link (acetal).

SUCROSE LACTOSE

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Tri- and tetrasaccharidesRaffinose

α-D-galactopyranosyl-(1 6)-α-D-glucopyranosyl-(1 2)-β-D-fructofuranose

Stachyoseα-D-galactopyranosyl-(1 6)- α-D-galactopyranosyl-(1 6)-α-D-glucopyranosyl-(1 2)-β-D-fructofuranose

• Both found in legumes (peas, beans, soya)• But neither hydrolysed nor digested by humans……• providing “a feast” for bacteria (e.g. E. coli) in the gut

which produce H2 and CO2 causing flatulence.

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Invert sugars• Invert sugar is produced by acid hydrolysis of sucrose• Produces a mixture of glucose and fructose

H2O + sucrose → glucose + fructose[α]D +66.5º +52.7 º - 92.4º

– Rotation of solution goes from dextrorotatory to laevorotatory overall

– Process is called “inversion” of sucrose - producing “invert sugar”

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Polysaccharides• Digestible

– Starches (glycogen / amylose / amylopectin)

• Indigestible (dietary fibre / roughage)– Cellulose– Pectin– Seaweed polysaccharides (agarose / carageenan)– Xanthan and gellan (gums)

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Glycogen• Glycogen is the major

carbohydrate storage molecule in animals.

• It is a homopolymer of glucose (a glucan) with α-(1,4)-linkages.

• It is also highly branched, with α-(1,6)branch linkages occurring every 8-10 residues.

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Starches: amylose and amylopectin

• Starch is the major form of stored carbohydrate in plant cells e.g. grains, potatoes etc.

• Unbranched starch is called amylose and branchedstarch is called amylopectin.– Amylose: linear glucan with α-(1,4)-glycosidic linkages

and ~104 subunits– Amylopectin: branched glucan with α−(1,4) and α-(1,6)

linkages with ~106 subunits: more compact than amylose for same MW

• Structure is identical to glycogen, except for a much lower degree of branching (about every 20-30 residues).

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Cellulose• Cellulose is a a linear glucan with β-(1,4)-glycosidic

linkages– Humans lack cellulase enzymes, so cellulose is

indigestible for us

• Different stereochemistry of glycosidic bond compared to starches [i.e. β−(1,4)- vs. α-(1,4)-] – results in very different molecular shape in solution

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Amylose vs. Cellulose

(Solomons and Fryhle, 2000; © John Wiley & Sons, Inc.)

Amylose helix Cellulose sheets

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Pectin• Polysaccharide of D-galacturonic acid (and derivatives)

joined by α-(1,4)-linkages

– can contain up to 20% neutral sugars– found naturally in fruits, vegetables– charged at higher pH– can form gels - used in the making of jams / preserves

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Other food polysaccharides

Polysaccharide Subunits Linkages Characteristics

Alginate D-mannuronic + L-guluronic

α−(1,4) + β−(1,4)

Charged, linear

Agarose D-galactose α−(1,3) + β−(1,4)

Neutral, linear

Carrageenan sulphonated D-galactose

α−(1,3) + β-(1,4)

Charged (-SO3-), linear

Guar Gum D-mannose & D-galactose

α−(1,6) + β−(1,4)

Neutral, branched